The consequences of the structural and functional homology of a family of guanine nucleotide binding proteins for their activation by hormone receptors will be investigated. These GTP binding proteins act as intermediaries between hormones receptor on the cell surface and intracellular second messenger systems. At least three different members of this family have been identified, and there are indications that there are additional such proteins. These proteins have a similar subunit composition and a number of common functional properties. There are at least four different second messenger functions affected by these proteins. (Specific Aim 1) Experiments will attempt to identify additional members of this family in various rat and bovine tissues based upon the common properties of the already identified proteins. Small quantities of new proteins will be purified and compared structurally and functionally with the other members of this family. (Specific Aim 2) The interactions of newly discovered and/or previously identified proteins with hormone receptors will be studied. Receptor interactions will be detected by hormone-induced subunit dissociation of the proteins and hormone induced guanine nucleotide binding. Initial studies will use human platelet membranes which contain a well characterized second messenger system for the synthesis of cAMP. This system is regulated by two of these proteins, one mediating stimulation by PGE1 and another inhibition by epinephrine. These functional studies will then be extended to various rat and bovine tissues containing different patterns of hormone receptors and GTP binding proteins. (Specific Aim 3) The tissue-specific distribution of those members of this family which are substrates for bacterial toxins in rat and bovine tissues will be characterized. Novel forms identified, or forms not previously amenable to isolation, will be purified and characterized. Preliminary studies indicate that testis may be an advantageous source of one such form of the GTP binding protein regulating stimulation of adenylate cyclase. There are indications that this form of the protein may preferentially mediate hormonal stimulation of adenylate cyclase.